U.S. patent application number 10/029989 was filed with the patent office on 2002-09-26 for surface acoustic wave device.
This patent application is currently assigned to Murata Manufacturing Co., Ltd.. Invention is credited to Takamine, Yuichi.
Application Number | 20020135267 10/029989 |
Document ID | / |
Family ID | 18871277 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020135267 |
Kind Code |
A1 |
Takamine, Yuichi |
September 26, 2002 |
Surface acoustic wave device
Abstract
A surface acoustic wave device includes a plurality of surface
acoustic wave filters having different center frequencies contained
in a package. In the surface acoustic wave device, one of the input
terminal or the output terminal of at least one of the plurality of
surface acoustic wave filters is a balanced terminal and the other
is an unbalanced terminal.
Inventors: |
Takamine, Yuichi;
(Kanazawa-shi, JP) |
Correspondence
Address: |
Keating & Bennett LLP
Suite 312
10400 Eaton Place
Fairfax
VA
22030
US
|
Assignee: |
Murata Manufacturing Co.,
Ltd.
Nagaokakyo-shi
JP
|
Family ID: |
18871277 |
Appl. No.: |
10/029989 |
Filed: |
December 31, 2001 |
Current U.S.
Class: |
310/313R |
Current CPC
Class: |
H03H 9/008 20130101;
H03H 9/14588 20130101; H03H 9/0085 20130101; H03H 9/0071 20130101;
H03H 9/6436 20130101; H03H 9/6423 20130101; H03H 9/0038 20130101;
H03H 9/1071 20130101; H03H 2250/00 20130101; H03H 9/0057
20130101 |
Class at
Publication: |
310/313.00R |
International
Class: |
H03H 009/46 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2001 |
JP |
2001-002838 |
Claims
What is claimed is:
1. A surface acoustic wave device comprising: a package; and a
plurality of surface acoustic wave filters having different center
frequencies and included in said package; wherein one of an input
terminal and an output terminal of at least one of said plurality
of surface acoustic wave filters is a balanced terminal and the
other of the input terminal and the output terminal of said at
least one of said plurality of surface acoustic wave filters is an
unbalanced terminal.
2. A surface acoustic wave device according to claim 1, wherein one
of the input terminal and the output terminal of each of said
plurality of surface acoustic wave filters is a balanced terminal
and the other of the input terminal and the output terminal of each
of said plurality of surface acoustic wave filters is an unbalanced
terminal.
3. A surface acoustic wave device according to claim 2, wherein the
unbalanced terminal is shared among said plurality of surface
acoustic wave filters.
4. A surface acoustic wave device according to claim 3, further
comprising an impedance matching element connected to the shared
unbalanced terminal.
5. A surface acoustic wave device according to claim 4, wherein
said impedance matching element is an inductor connected in
parallel to the unbalanced terminal.
6. A surface acoustic wave device according to one of claim 1,
wherein at least one of said plurality of surface acoustic wave
filters is a surface acoustic wave filter including cascaded
resonators.
7. A surface acoustic wave device according to one of claim 1,
wherein at least one of said plurality of surface acoustic wave
filters has a different electrode thickness from the other surface
acoustic wave filters.
8. A surface acoustic wave device according to claim 7, wherein
each of said plurality of surface acoustic wave filters is disposed
on a single piezoelectric substrate.
9. A surface acoustic wave device according to claim 1, wherein one
of the plurality of surface acoustic wave filters is constructed to
perform DCS reception and another of the surface acoustic wave
filters is constructed to perform PCS reception.
10. A surface acoustic wave device according to claim 1, wherein
the plurality of surface acoustic wave filters are integral with
each other.
11. A surface acoustic wave device according to claim 1, further
comprising a single substrate, wherein said plurality of surface
acoustic wave filters are defined by electrodes disposed on said
single substrate.
12. A surface acoustic wave device according to claim 11, wherein
said single substrate is a 40.+-.5.degree. Y-cut X-propagating
LiTaO.sub.3 substrate.
13. A surface acoustic wave device according to claim 1, further
comprising a single substrate, wherein said plurality of surface
acoustic wave devices mounted face-down on the single
substrate.
14. A surface acoustic wave device according to claim 1, wherein
said package includes a base substrate and a surrounding sidewall
fixed on the base substrate.
15. A surface acoustic wave device according to claim 1, wherein
each of the surface acoustic wave filters is constructed to perform
a function of unbalanced/balanced conversion.
16. A surface acoustic wave device according to claim 1, further
comprising an inductor connected between the balanced output
terminals.
17. A surface acoustic wave device according to claim 16, further
comprising a substrate on which said plurality of surface acoustic
waves are provided, wherein the inductor is provided within the
package or on the substrate.
18. A surface acoustic wave device according to claim 1, further
comprising an inductor connected to the package.
19. A surface acoustic wave device according to claim 18, further
comprising a substrate on which said plurality of surface acoustic
waves are provided, wherein the inductor is provided within the
package or on the substrate.
20. A communications device comprising a surface acoustic wave
device according to claim 1.
21. The communications device according to claim 20, wherein said
surface acoustic wave device defines a band-pass filter.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to surface acoustic wave
devices, and more specifically, to a surface acoustic wave device
including a plurality of surface acoustic wave filters contained in
a single package.
[0003] 2. Description of the Related Art
[0004] In recent years, multiband cellular phones compatible with
two or more communications systems have been designed with the aim
of improving functionality of mobile communications devices. A
cellular phone of this type requires a broadband filter. However,
it has been difficult to implement a broadband filter which covers
two or more bands and which causes only small loss.
[0005] Accordingly, surface acoustic wave devices incorporating a
plurality of surface acoustic wave filters contained in a single
package, as shown in FIGS. 18 and 19, have been widely used.
[0006] In the surface acoustic wave device 100 shown in FIG. 18,
surface acoustic wave filters 101 and 102 having different center
frequencies are contained in a package 103. In the surface acoustic
wave filter 101, a signal is input to an unbalanced input terminal
104, and a signal is output from an unbalanced output terminal 105.
Similarly, in the surface acoustic wave filter 102, a signal is
input from an unbalanced input terminal 106, and a signal is output
from an unbalanced output terminal 107.
[0007] That is, the surface acoustic wave device shown in FIG. 18
outputs two unbalanced output signals corresponding to two
unbalanced input signals.
[0008] In another surface acoustic wave device of the related art,
a surface acoustic wave device 200 shown in FIG. 19, two surface
acoustic wave filters 201 and 202 having different center
frequencies are contained in a package 203. The surface acoustic
wave filters 201 and 202 share a common unbalanced input terminal
204. That is, the surface acoustic wave filters 201 and 202 receive
a signal input to the unbalanced input terminal 204, and output
signals respectively to unbalanced output terminals 205 and 206. In
this example, because the impedance on the side of the common
unbalanced input terminal 204 is capacitive, an inductor 207 is
connected in parallel to the unbalanced input terminal 204. The
inductor 207 is arranged internally or externally with respect to
the package 203. A surface acoustic wave device of this type is
disclosed, for example, in Japanese Unexamined Patent Application
Publication No. 11-68511.
[0009] When two surface acoustic wave filters with unbalanced input
and output terminals, having different center frequencies, are
contained in a single package, there has been a problem that
stopband attenuation is reduced due to electromagnetic or
capacitive mutual interference between the two surface acoustic
wave filters, which results in the filter failing to provide a
sufficient attenuation. The mutual interference can be reduced to a
certain extent when the electrical interconnection between the
surface acoustic wave filters and electrode pads of the package is
achieved by wire bonding, by optimizing the layout of the electrode
pads, bonding conditions, and other methods. However, it has been
difficult to reduce the mutual interference particularly when the
surface acoustic wave filters are electrically connected to and
contained in the package by the face-down manufacturing method
compared with the case of wire bonding.
SUMMARY OF THE INVENTION
[0010] In order to overcome the problems described above, preferred
embodiments of the present invention provide a surface acoustic
wave device including a plurality of surface acoustic wave filters
contained in a single package, in which the effect of mutual
interference between the plurality of surface acoustic wave filters
is minimized so as to provide a sufficient stopband
attenuation.
[0011] According to one preferred embodiment of the present
invention, a surface acoustic wave device includes a package, and a
plurality of surface acoustic wave filters having different center
frequencies and contained in the package, wherein one of the input
terminal and the output terminal of at least one of the plurality
of surface acoustic wave filters is a balanced terminal and the
other of the input terminal and the output terminal is an
unbalanced terminal.
[0012] As a result of this unique arrangement, stopband attenuation
is increased compared with the surface acoustic wave device 100
according to the related art shown in FIG. 18 in which a plurality
of surface acoustic wave filters with an unbalanced input and an
unbalanced output is contained in a single package.
[0013] Preferably, in the surface acoustic wave device, one of the
input terminal or the output terminal of each of the plurality of
surface acoustic wave filters is a balanced terminal and the other
is an unbalanced terminal.
[0014] Accordingly, stopband attenuation is increased for all of
the surface acoustic wave filters included in the surface acoustic
wave device.
[0015] Furthermore, the unbalanced terminal may be shared among the
plurality of surface acoustic wave filters.
[0016] Accordingly, stopband attenuation is increased compared with
the surface acoustic wave device 200 with a single unbalanced input
and two unbalanced outputs according to the related art shown in
FIG. 19.
[0017] The surface acoustic wave device according to another
preferred embodiment of the present invention preferably includes
an impedance matching element attached to the shared unbalanced
terminal.
[0018] Preferably, the impedance matching element is an inductor
attached in parallel to the unbalanced terminal. Accordingly,
frequency characteristics are even more improved.
[0019] Furthermore, at least one of the plurality of surface
acoustic wave filters is preferably a surface acoustic wave filter
of the type using cascaded resonators.
[0020] Furthermore, at least one of the plurality of surface
acoustic wave filters preferably has a different electrode
thickness from the other surface acoustic wave filters.
[0021] Accordingly, the filter characteristics of each of the
surface acoustic wave filters can be optimized by varying the
electrode thicknesses thereof, so as to achieve favorable frequency
characteristics.
[0022] The plurality of surface acoustic wave filters may be
disposed on a single piezoelectric substrate.
[0023] Accordingly, the size of the surface acoustic wave device
can be minimized.
[0024] According to another preferred embodiment of the present
invention, a communications device includes a surface acoustic wave
device according to preferred embodiments described above, which
surface acoustic wave device defines a band-pass filter.
[0025] Other features, elements, characteristics and advantages of
the present invention will become more apparent from the following
detailed description of preferred embodiments with reference to the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic construction diagram of a surface
acoustic wave device according to a first preferred embodiment of
the present invention;
[0027] FIG. 2 is a schematic sectional front view of the surface
acoustic wave device according to the first preferred embodiment of
the present invention;
[0028] FIG. 3 is a schematic plan view showing the structure of
electrodes in one of the surface acoustic wave filters incorporated
in the surface acoustic wave device according to the first
preferred embodiment of the present invention;
[0029] FIG. 4 is a schematic plan view showing the structure of
electrodes in the other surface acoustic wave filter incorporated
in the surface acoustic wave device according to the first
preferred embodiment of the present invention;
[0030] FIG. 5 is a graph showing the frequency characteristics of a
surface acoustic wave filter for DCS reception, which is used as
one of the surface acoustic wave filters incorporated in the
surface acoustic wave device according to the first preferred
embodiment of the present invention;
[0031] FIG. 6 is a graph showing the frequency characteristics of a
surface acoustic wave filter for DCS reception incorporated in a
surface acoustic wave device according to a related art;
[0032] FIG. 7 is a graph showing the frequency characteristics of a
surface acoustic wave filter for PCS reception, which is used as
the other surface acoustic wave filter incorporated in the surface
acoustic wave device according to the first preferred embodiment of
the present invention;
[0033] FIG. 8 is a graph showing the frequency characteristics of a
surface acoustic wave filter for PCS reception incorporated in the
surface acoustic wave device according to the related art;
[0034] FIG. 9 is a schematic plan view showing the electrode
structure of the surface acoustic wave filter for DCS reception
incorporated in the surface acoustic wave device according to the
related art;
[0035] FIG. 10 is a schematic plan view showing the electrode
structure of the surface acoustic wave filter for PCS reception
incorporated in the surface acoustic wave device according to the
related art;
[0036] FIG. 11 is a schematic plan view showing a surface acoustic
wave filter of the type using cascaded resonators as an example of
surface acoustic wave filter used to implement various preferred
embodiments of the present invention;
[0037] FIG. 12 is a schematic plan view showing a surface acoustic
wave filter of the type using cascaded resonators as another
example of surface acoustic wave filter used to implement various
preferred embodiments of the present invention;
[0038] FIG. 13 is a schematic plan view showing a surface acoustic
wave filter of the type using cascaded resonators as still another
example of surface acoustic wave filter used to implement various
preferred embodiments of the present invention;
[0039] FIG. 14 is a schematic plan view showing a surface acoustic
wave filter of the type using cascaded resonators as yet another
example of surface acoustic wave filter used to implement various
preferred embodiments of the present invention;
[0040] FIG. 15 is a schematic construction diagram of a surface
acoustic wave device according to a second preferred embodiment of
the present invention;
[0041] FIG. 16 is a schematic construction diagram showing a
modification of the surface acoustic wave device according to the
second preferred embodiment of the present invention;
[0042] FIG. 17 is a schematic block diagram showing the
construction of a communications device incorporating a surface
acoustic wave filter according to various preferred embodiments of
the present invention;
[0043] FIG. 18 is a schematic construction diagram of a surface
acoustic wave device according to the related art; and
[0044] FIG. 19 is a schematic construction diagram of a surface
acoustic wave device according to another related art.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0045] The present invention will be described with reference to
preferred embodiments thereof and with reference to the
accompanying drawings.
[0046] FIG. 1 schematic construction diagram of a surface acoustic
wave device according to a first preferred embodiment of the
present invention. The surface acoustic wave device according to
the first preferred embodiment preferably includes a surface
acoustic wave filter 1 for DCS reception and a surface acoustic
wave filter 2 for PCS reception included in a single package 3. The
surface acoustic wave filters 1 and 2 are preferably integral and
are defined by electrodes disposed on a single piezoelectric
substrate. As the piezoelectric substrate, a 40.+-.5.degree. Y-cut
X-propagating LiTaO.sub.3 substrate is preferably used. Electrodes
including interdigital transducers constituting the surface
acoustic wave filters 1 and 2 are preferably made of Al.
[0047] FIG. 2 is a schematic front sectional view of the surface
acoustic wave device according to the first preferred embodiment of
the present invention.
[0048] The surface acoustic wave filters 1 and 2 (not shown in FIG.
2) are implemented using the piezoelectric substrate 11 preferably
made of LiTaO.sub.3.
[0049] The surface acoustic wave device is contained in the package
3 and is preferably mounted via the face-down manufacturing method.
That is, the surface of the piezoelectric substrate 11 on which the
electrodes constituting the surface acoustic wave filters 1 and 2
are arranged to be facing down.
[0050] The package 3 includes a base substrate 3a and a surrounding
sidewall 3b fixed on the base substrate 3a. The upside opening of
the sidewall 3b is covered by a lid member 3c. Furthermore, on the
base substrate 3a, an electrode land 3d for attaching dies is
provided. The surface acoustic wave device is joined to the
electrode land 3d via bumps 12, and the electrodes of the surface
acoustic wave device are electrically connected to the electrode
land 3d of the package 3.
[0051] Referring back to FIG. 1, the surface acoustic wave filter 1
includes an unbalanced input terminal 4, and balanced output
terminals 5 and 6. The surface acoustic wave filter 2 includes an
unbalanced input terminal 7, and balanced output terminals 8 and
9.
[0052] That is, each of the surface acoustic wave filters 1 and 2
performs a function of unbalanced/balanced conversion. The input
terminal and the output terminals may be used in the reverse
manner.
[0053] Between the balanced output terminals 5 and 6, an inductor
10a with an inductance of approximately 18 nH is attached
externally to the package 3. Between the balanced output terminals
8 and 9, an inductor 10b with an inductance of approximately 27 nH
is attached externally to the package 3. The inductors 10a and 10b
may be disposed on the piezoelectric substrate 11 on which the
surface acoustic wave filters 1 and 2 are provided or may be
disposed internally within the package 3.
[0054] According to this preferred embodiment, in both of the
surface acoustic wave filters 1 and 2, stopband attenuation is
improved compared with the surface acoustic wave devices according
to the related arts in which two surface acoustic wave filters are
contained in a single package. This will be described based on an
actual experiment.
[0055] FIG. 3 shows the structure of the electrodes in the surface
acoustic wave filter 1, and FIG. 4 shows the structures of the
electrodes in the surface acoustic wave filter 2 in the present
preferred embodiment.
[0056] The surface acoustic wave filter 1 used in the experiment
included two surface acoustic wave filters 501 and 502 of the type
using cascaded resonators, and four surface acoustic wave
resonators 503 to 506. The surface acoustic wave filter 501
included first to third IDTs 507 to 509. The IDTs 507 and 509 were
disposed on both sides of the middle IDT 508 in the direction of
propagation of surface acoustic waves. Furthermore, reflectors 510
and 511 were disposed on both sides of the area where the IDTs 507
to 509 were arranged in the direction of propagation of surface
acoustic waves. In FIG. 3, for simplicity, the number of the
electrode fingers is shown as smaller than the actual number. The
specific design of the surface acoustic wave filter 501 was as
follows:
[0057] Overlap width of electrode fingers: W=41.7.lambda.I;
[0058] Number of electrode fingers of IDTs: 22 for IDT 507, 31 for
IDT 508, and 22 for IDT 509;
[0059] Wavelength of IDTs: .lambda.I=2.16 .mu.m;
[0060] Wavelength of reflectors .lambda.R=2.20 .mu.m;
[0061] Number of electrode fingers of reflectors: 120;
[0062] Distance between IDTs: 0.76.lambda.I;
[0063] Distance between IDT and reflector: 0.51.lambda.R;
[0064] Duty ratio of IDTs: 0.60;
[0065] Duty ratio of reflectors: 0.60;
[0066] Thickness of electrodes in IDTs and reflectors:
0.08.lambda.I As is apparent from FIG. 3, electrode fingers 508 a
and 508b on both ends of the second IDT 508 in the middle in the
direction of propagation of surface acoustic waves were made wider
than the other electrode fingers, so that free spaces between IDTs
were made smaller. The surface acoustic wave filter 502 was
basically the same as the surface acoustic wave filter 501.
However, in order to reverse the phase of signals, the distance
between IDTs was made wider by about 0.5.lambda.I to be
approximately 1.26.lambda.I.
[0067] The specific design of the surface acoustic wave resonators
503 to 506 is shown in Table 1 below.
1 TABLE 1 503 and 504 505 and 506 Overlap width 19.0.lambda.
16.6.lambda. Number of IDTs 241 241 Wavelength .lambda. (both 2.11
.mu.m 2.11 .mu.m IDTs and reflectors) Distance between IDT
0.50.lambda. 0.50.lambda. and reflector Number of reflectors 30 30
Duty (both IDTs and 0.60 0.60 reflectors) Electrode thickness
0.082.lambda. 0.082.lambda.
[0068] The surface acoustic wave filter 2 included two surface
acoustic wave filters 601 and 602 of the type using cascaded
resonators, and four surface acoustic wave resonators 603 to 606,
as shown in FIG. 4. The surface acoustic wave filter 601 included
first to third IDTs 607 to 609 arranged in the direction of
propagation of surface acoustic waves. Furthermore, reflectors 610
and 611 were formed on both sides of the area where the IDTs 607 to
609 were formed in the direction of propagation of surface acoustic
waves. In FIG. 4, for simplicity, the number of electrode fingers
is shown as smaller than the actual number.
[0069] The specific design of the surface acoustic wave filter 601
was as follows:
[0070] Overlap width: W=29.7.lambda.I;
[0071] Number of electrode fingers of IDTs: 24 for IDT 607, 35 for
IDT 608, and 24 for IDT 609;
[0072] Wavelength of IDTs: .lambda.I=2.02 .mu.m;
[0073] Wavelength of reflectors .lambda.R=2.05 .mu.m;
[0074] Number of electrode fingers of reflectors: 100;
[0075] Distance between IDTs: 0.79.lambda.I;
[0076] Distance between IDT and reflector: 0.52.lambda.R;
[0077] Duty ratio of IDTs: 0.60;
[0078] Duty ratio of reflectors: 0.60;
[0079] Electrode thickness of IDTs and reflectors:
0.08.lambda.I
[0080] Electrode fingers 608a and 608b on both ends of the IDT 608
in the direction of propagation of surface acoustic waves were made
wider than the other electrode fingers, so that free space between
the IDTs was made smaller. The surface acoustic wave filter 602 was
basically the same as the surface acoustic wave filter 601.
However, in order to reverse the phase of signals, the distance
between IDTs was made wider by about 0.5.lambda.I to be
approximately 1.29.lambda.I.
[0081] The specific design of the surface acoustic wave resonators
603 to 606 is shown in Table 2 below.
2 TABLE 2 603 and 604 605 and 606 Overlap width 19.8.lambda.
34.7.lambda. Number of IDTs 281 281 Wavelength .lambda. (both IDTs
2.02 .mu.m 2.02 .mu.m and reflectors) Distance between IDT and
0.50.lambda. 0.50.lambda. reflector Number of reflectors 30 30 Duty
(both IDTs and 0.60 0.60 reflectors) Electrode thickness
0.080.lambda. 0.080.lambda.
[0082] As described above, the electrode thickness of IDTs in the
surface acoustic wave filters 501, 502, 601, and 602 was about 8%
of the wavelength of the IDTs in both of the surface acoustic wave
filters 1 and 2. Thus, the absolute electrode thickness differed
between the two surface acoustic wave filters 1 and 2.
[0083] Although the two surface acoustic wave filters 1 and 2 with
different electrode thicknesses were disposed on the single
piezoelectric substrate 11 in this example of the present preferred
embodiment, the surface acoustic wave filters 1 and 2 may be
disposed on separate piezoelectric substrates, that is, two
piezoelectric substrates for defining the surface acoustic wave
filters 1 and 2 thereon may be contained in a single package.
[0084] FIG. 5 shows the frequency characteristics of the surface
acoustic wave filter 1 for DCS reception, and FIG. 7 shows the
frequency characteristics of the surface acoustic wave filter 2 for
PCS reception in this preferred embodiment. For comparison, FIG. 6
shows the frequency characteristics of the surface acoustic wave
filter for DCS reception, and FIG. 8 shows the frequency
characteristics of the surface acoustic wave filter for PCS
reception in the surface acoustic wave device 100 according to the
related art shown in FIG. 18, in which two surface acoustic wave
filters with unbalanced input and unbalanced output are contained
in a single package.
[0085] In implementing the surface acoustic wave device 100
according to the related art, the structure of electrodes in the
surface acoustic wave filters 101 and 102 were as shown in FIGS. 9
and 10, respectively. More specifically, the surface acoustic wave
filter 101 included a surface acoustic wave filter 701 of the type
using cascaded resonators, and surface acoustic wave resonators 702
and 703. In the surface acoustic wave filter 701, IDTs 704 to 706
were arranged in the direction of propagation of surface acoustic
waves, and reflectors 707 and 708 were arranged on both sides of
the area where the IDTs 704 to 706 were located.
[0086] The specific design of the surface acoustic wave filter 701
was the same as the surface acoustic wave filter 501 in the surface
acoustic wave device according to the present preferred embodiment,
except that the overlap width was doubled. Also, the design of the
surface acoustic wave resonators 702 and 703 were the same as the
surface acoustic wave resonators 503 and 505 in the surface
acoustic wave device according to the present preferred embodiment,
except that the overlap width was doubled.
[0087] The implementation of the surface acoustic wave filter 102
in the surface acoustic wave device 100 according to the related
art included a surface acoustic wave filter 801 of the type using
cascaded resonators, and two surface acoustic wave resonators 802
and 803, as shown in FIG. 10. The specific design of the surface
acoustic wave filter 801 was the same as the surface acoustic wave
filter 601 in the present preferred embodiment, except that the
overlap width was doubled. Also, the surface acoustic wave
resonators 802 and 803 were the same as the surface acoustic wave
resonators 603 and 605 in the surface acoustic wave device
according to the present preferred embodiment, except that the
overlap width of electrode fingers was doubled.
[0088] The present preferred embodiment and the related art of FIG.
18 differed in that the input impedance and the output impedance
were 50 .OMEGA. in the surface acoustic wave device according to
the related art, whereas the impedance on the unbalanced input side
was 50 .OMEGA. and the impedance on the balanced output side was
200 .OMEGA. in the example of the present preferred embodiment.
[0089] By comparing the characteristics shown in FIG. 5 and FIG. 6
and the characteristics shown in FIG. 7 and FIG. 8, respectively,
it will be understood that stopband attenuation was improved in
each of the surface acoustic wave filters 1 and 2 in the surface
acoustic wave device according to the present preferred embodiment
compared with the related art. More specifically, for example, the
minimum attenuation in a frequency range of about 0 GHz to about 1
GHz was 42 dB in the surface acoustic wave filter for DCS reception
in the related art, whereas it was about 55 dB in the example of
the present preferred embodiment, achieving an improvement of about
13 dB. Furthermore, as for the surface acoustic wave filter for PCS
reception, the minimum attenuation was 32 dB in the surface
acoustic wave device according to the related art, whereas it was
about 47 dB in the example of the present preferred embodiment,
achieving an improvement of about 15 dB. Furthermore, comparing the
minimum attenuation in a frequency range of about 4 GHz to about 6
GHz in the filter for DCS reception, the minimum attenuation was 18
dB in the related art, whereas it was about 35 dB in the example of
the present preferred embodiment, achieving an improvement of about
17 dB. Similarly, as for the filter for PCS reception, the minimum
attenuation was 23 dB in the related art, whereas it was about 42
dB in the present preferred embodiment, achieving an improvement of
about 19 dB.
[0090] The reason the stopband attenuation was improved in the
example of the present preferred embodiment as described above will
be described below. The stopband attenuation in a surface acoustic
wave device with unbalanced input (output) and balanced output
(input), such as in the present preferred embodiment, is
significantly affected by the balance of filters. The balance of
filters is represented by the difference in the amplitude and phase
of the transmission characteristics between the unbalanced terminal
and the balanced terminal, respectively referred to as amplitude
balance and phase balance.
[0091] Assuming that the surface acoustic wave filter with
unbalanced input (output) and balanced output (input) is a
three-port device, for example, the unbalanced input terminal being
port 1 and the pair of balanced output terminals being ports 2 and
3, the amplitude balance
.vertline.A.vertline.=.vertline.20log(S21).vertline.-.vertline.20log(S31)-
.vertline., and the phase balance
.vertline.B.vertline.=.vertline..angle.S- 21-.angle.S31, wherein
S21 is a transfer factor from port 1 to port 2, and S31 is a
transfer factor from port 1 to port 3.
[0092] Ideally, the amplitude balance is 0 dB and the phase balance
is 0 degrees in the stopband. The stopband attenuation of a filter
having an ideal balance is infinite. That is, the stopband
attenuation of a surface acoustic wave filter with unbalanced input
and balanced output becomes larger compared with a surface acoustic
wave filter with unbalanced input and unbalanced output as the
balance becomes more approximate to the ideal balance.
[0093] Although a 40.+-.5.degree. Y-cut X-propagating LiTaO.sub.3
substrate is preferably used in the example of the preferred
embodiment, since the present invention is intended to improve the
balance of the plurality of the surface acoustic wave filters 1 and
2 and to improve stopband attenuation by providing a function of
unbalanced/balanced conversion, the piezoelectric substrate is not
limited thereto, and other types of substrates, such as a
64.degree. to 72.degree. Y-cut X-propagating LiNbO.sub.3 substrate
and a 41.degree. Y-cut X-propagating LiNbO.sub.3 substrate, may be
used.
[0094] Furthermore, in the present preferred embodiment, for
example, although the surface acoustic wave filter 1 with
unbalanced input and balanced output was implemented using the two
surface acoustic wave filters 501 and 502, the construction of the
surface acoustic wave filter 1 is not limited thereto as long as it
has an unbalanced input and balanced output.
[0095] For example, as shown in FIG. 11, the arrangement may be
such that a surface acoustic wave filter 901 of the type using
cascaded resonators includes first to third IDTs 902 to 904, and
reflectors disposed on both sides of the area where the IDTs 902 to
904 are located in the direction of propagation of surface acoustic
waves, and the phase of the IDTs 902 and 904 on both sides is
reversed relative to the IDT 903, so that a function of
unbalanced/balanced conversion is provided. In this example, an
unbalanced input terminal 905 is connected to the IDT 903, and
balanced output terminals 906 and 907 are connected to the IDTs 902
and 904, respectively.
[0096] Furthermore, as shown in FIG. 12, the arrangement may be
such that two surface acoustic wave filters 1001 and 1002 of the
type using cascaded resonators, in which the phase of the middle
IDT is opposite relative to the IDTs on both sides, are connected
in parallel, and another surface acoustic wave filter 1003 of the
type using cascaded resonators is connected in series. In this
example, one end of the surface acoustic wave filter 1003 is
connected to an unbalanced input terminal 1004, and the middle IDTs
of the surface acoustic wave filters 1001 and 1002 are connected to
balanced output terminals 1005 and 1006, respectively.
[0097] Furthermore, as shown in FIG. 13, the arrangement may be
such that the overlap width of the surface acoustic wave filter
1003 shown in FIG. 12 is reduced to one half, the surface acoustic
wave filter 1003 is divided into two surface acoustic wave filters
1101 and 1102 of the type using cascaded resonators, and the
surface acoustic wave filters 1101 and 1102 are respectively
connected in series to the surface acoustic wave filters 1001 and
1002. Furthermore, as shown in FIG. 14, the arrangement may be such
that two surface acoustic wave filters 1201 and 1202 of the type
using cascaded resonators are cascaded to define two stages, the
middle IDT 1203 of the surface acoustic wave filter 1202 is divided
into two, and the two IDT segments are connected respectively to
balanced output terminals 1204 and 1205, so that balanced signals
will be obtained.
[0098] That is, according to preferred embodiments of the present
invention, the construction of the electrode fingers of the surface
acoustic wave filter having a function of unbalanced/balanced
conversion is not limited specifically, and any of the above
modifications achieves an improvement in balance similarly to the
preferred embodiments of the present invention, thereby improving
stopband attenuation.
[0099] Although the filter for DCS reception and the filter for PCS
reception are contained in a single package in preferred
embodiments described above, according to the present invention,
three or more surface acoustic wave filters may be contained in a
single package. Furthermore, according to preferred embodiments of
the present invention, various surface acoustic wave filters other
than those used for DCS reception and PCS reception may also be
contained in a single package. For example, a filter for EGSM
reception and a filter for DCS reception may be contained in a
single package, or a surface acoustic wave filter for EGSM
reception, a surface acoustic wave filter for DCS reception, and a
surface acoustic wave filter for PCS reception may be contained in
a single package.
[0100] FIG. 15 is a schematic block diagram showing the
construction of a surface acoustic wave device according to a
second preferred embodiment of the present invention. In the
surface acoustic wave device according to the second preferred
embodiment, a surface acoustic wave filter 1301 for DCS reception
and a surface acoustic wave filter 1302 for PCS reception
preferably include Al electrodes disposed on a single piezoelectric
substrate (not shown). The piezoelectric substrate is preferably
the same as in the first preferred embodiment. Similarly to the
first preferred embodiment, a surface acoustic wave device
including the surface acoustic wave filters 1301 and 1302 are
mounted in a package 1303 by the face-down manufacturing
method.
[0101] The second preferred embodiment differs from the first
preferred embodiment in that the surface acoustic wave filters 1301
and 1302 share a common unbalanced input terminal 1304, and an
inductor 1309 is attached in parallel to the unbalanced input
terminal 1304. The construction on the side of balanced output
terminals 1305, 1306, 1307, and 1308 is the same as in the first
preferred embodiment of the present invention.
[0102] Even if the unbalanced input signal terminal is shared as in
the second preferred embodiment, because the output terminals of
the surface acoustic wave filters 1301 and 1302 are balanced,
stopband attenuation is improved compared with the surface acoustic
wave device according to the related art shown in FIG. 19.
[0103] Furthermore, compared with the related art shown in FIG. 18,
although the related art shown in FIG. 19 has had the advantage
that the shared input terminal eliminates the need for a switch for
switching between two signal paths in the circuitry, the related
art shown in FIG. 19 has suffered the problem that the impedance
matching element 207 is required and therefore, for example,
impedance matching elements 208 and 209 must be provided on the
output side, the attenuation thus being reduced due to
electromagnetic coupling between the impedance matching elements
208 and 209. As opposed to the related art shown in FIG. 19, in the
surface acoustic wave device according to the second preferred
embodiment, the output terminals 1305, 1306, 1307, and 1308 are
balanced, so that common-mode signals due to electromagnetic
coupling is cancelled, thereby improving stopband attenuation.
[0104] In the surface acoustic wave device according to the second
preferred embodiment of the present invention, the specific
constructions of the surface acoustic wave filters 1301 and 1302
may be the same as the surface acoustic wave device 200 according
to the related art shown in FIG. 19. More specifically, impedance
matching can be readily achieved by connecting a surface acoustic
wave resonator in series between the common unbalanced input
terminal 1304 and the surface acoustic wave filters 1301 and 1302.
Impedance matching can be further facilitated by attaching the
surface acoustic wave resonator between at least the surface
acoustic wave filter having the relatively highest center frequency
and the common unbalanced input terminal 1304. Furthermore,
attenuation in the proximity of the pole in the higher side of the
passband can be further increased by making the anti-resonance
frequency of the surface acoustic wave higher than the higher side
of the passband of the surface acoustic wave filters.
[0105] Furthermore, the sharpness of attenuation in the higher
range of the passband can be further improved by withdrawing and
weighting the surface acoustic wave resonator, or making the
electrode thickness of the surface acoustic wave resonator thinner
than that of the surface acoustic wave filters.
[0106] Although the inductor 1309 is connected in parallel to the
unbalanced input terminal 1304 in the second preferred embodiment,
an impedance matching element other than the inductor 1309 may be
used. Furthermore, as shown in FIG. 16, the arrangement may be such
that the balanced output terminals of the surface acoustic wave
filters 1301 and 1302 are connected in parallel, so that the
unbalanced input terminal 1304 and the pair of balanced output
terminals 1305 and 1306 constitute a surface acoustic wave device
having two passbands.
[0107] FIG. 17 is a block diagram of a communications apparatus 160
according to another preferred embodiment of the present invention,
which includes a surface acoustic wave device according to other
preferred embodiments of the present invention.
[0108] Referring to FIG. 17, a diplexer 162 is connected to an
antenna 161. Between the diplexer 162 and receiver mixers 163 and
163a, a switch SW, a surface acoustic wave filter 164 and
amplifiers 165 and 165a constituting an RF stage are connected.
Furthermore, surface acoustic wave filters 169 and 169a
constituting an IF stage are connected to the mixers 163 and 163a,
respectively. Furthermore, between the diplexer 162 and a
transmitter mixer 166, an amplifier 167 and a surface acoustic wave
filter 168 constituting an RF stage are connected.
[0109] A surface acoustic wave device according to other preferred
embodiments of the present invention described above can be
suitably used as the surface acoustic wave filter 164 in the
communications apparatus 160.
[0110] While preferred embodiments of the invention have been
described above, it is to be understood that variations and
modifications will be apparent to those skilled in the art without
departing the scope and spirit of the invention. The scope of the
invention, therefore, is to be determined solely by the following
claims.
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